Atomic nuclei placed in a magnetic field precess about the magnetic field vector at a frequency proportional to the magnetic field strength and the gyromagnetic ratio, which is a fundamental constant for each nuclear species. Thus, for a given nucleus, the frequency of nuclear precession is solely a function of the applied magnetic field, which can be controlled. For hydrogen nuclei, which are commonly used as the diagnostic nuclei in MRSI, the precession frequency is approximately 64 MHz at a field strength of 1.5 Tesla.
Synchronized precessing nuclei produce a macroscopic net magnetization which is the principal effect by which data is collected in magnetic resonance techniques. The net magnetization rotates at the precession frequency and therefore induces an electrical signal in a pickup coil located in proximity to the precessing nuclei. This process is called free inductive decay (FID) and is necessary for performing magnetic resonance imaging. The principal procedure used in magnetic resonance imaging is inducing the synchronization of the nuclear magnetic moments of specific nuclear species (and specific to the molecules they are in) in a small, well defined volume of space. A series of radio frequency (RF) pulses applied in concert with magnetic field gradients is used to cause this synchronization. Each RF pulse can be spatially selective and affects the nuclear spins in a specific volume of space. After a final RF refocusing pulse is applied, the nuclear spins inside a volume of interest will be in phase, and a FID spin echo signal from the resultant rotating magnetization can be captured. Only the volume of interest will have a rotating magnetization, and therefore the FID signal captured will be specific to this volume. Using sequential excitation pulses in combination with phase encoding gradients allows voxels inside the volume of interest to be localized, providing higher spatial resolution.
When hydrogen atoms are incorporated into molecules, the surrounding electron clouds affect the magnetic field strength experienced by the hydrogen nuclei. This results in a small change in the precession frequency of the hydrogen nuclei, typically on the order of 100-300 Hz for a 1.5T field as compared to the hydrogen nuclei in water, which is a standard reference. This chemical shift is different for different chemicals and allows researchers to identify and locate various chemicals and their concentrations inside the body. Fourier transform techniques, for example, are often used to calculate a chemical shift spectrum for the FID signal from each voxel. In other words, each FID signal is decomposed into its frequency components with each frequency corresponding to a component of a specific chemical. It is often desirable to simultaneously detect several chemicals in the FID signal from each voxel.
One problem with MRSI using hydrogen nuclei is the prevalence of water and lipids in the human body. Water and lipids contain large amounts of hydrogen, which can produce very powerful FID signals that can mask FID signals from chemicals of interest which are often present in lower concentrations. Choline, lactate, and creatine are examples of chemicals which have diagnostic value but are present in the body in concentrations much lower than water or lipids. For these reasons, a useful MRS/MRSI technique should be able to provide high suppression for the FID signals from water and lipids. Further, the technique should be able to simultaneously detect the FID signals from several chemicals of interest while providing for water and lipid signal suppression. Water/lipid suppression is also referred to as solvent suppression in the arts of MRS and MRSI. Improvements are needed in the art because there are circumstances in which the most commonly used techniques for water suppression (chemical shift selective (CHESS) saturation) and lipid suppression (short-time inversion recovery (STIR)) may be too sensitive to T1 or local RF magnetic field variations to be adequate for many applications.
The technique used in one of many versions of MRS/MRSI for voxel localization is to apply a sequence of three frequency selective RF excitation/refocusing pulses synchronized with magnetic field gradients in three orthogonal directions. Each RF pulse excites nuclear moments having precession frequencies located within a predetermined bandwidth. Refer now to FIG. 1. For each pulse, the bandwidth of the pulse selects a planar region 18 of space due to the magnetic field gradient applied during the pulse. Each plane 18 is perpendicular to one of the three magnetic field gradients applied during the three RF pulses. At the end of a three-pulse sequence, a voxel 16 located at the intersection of the three planar, orthogonal regions 18 has been selected. Due to the chemical shifts of different chemicals, the voxels for different chemicals will be displaced in all 3orthogonal directions with respect to one another. FIG. 2A illustrates the chemical shift induced spatial displacements for water 20 and lipids 22, which have rather widely separated chemical shifts. Both the water 20 and lipid 22 excitation profiles are displaced with respect to the chemical shift imaging grid 23 (CSI grid). The CSI grid 23 serves as a reference for the construction of images, so it is important to have the chemicals of interest accurately located with respect to the CSI grid 23. Only one specific chemical shift on resonance 24 is not displaced with respect to the CSI grid, however. FIG. 2B illustrates the chemical shift induced spatial displacement in 2 dimensions for choline 25A and lactate 25B. The chemical shift induced spatial displacement maps the chemical shift into spatial displacement in all three orthogonal directions. It would be a significant improvement in MRS/MRSI technology to be able to reduce or eliminate the chemical shift induced spatial displacement.
The chemical shift induced spatial displacement also inhibits the accurate quantification of chemical concentrations and chemical concentration ratios. This is a problem because such chemical concentrations and concentration ratio measurements can be a key factor in distinguishing healthy tissue from diseased tissue. Lactate, for example, may be a marker indicating the presence of active cancer. FIG. 3 shows a CSI grid 23 of 16voxels with the choline (Cho), N-acetyl-L-aspartate (NAA), and lactate (Lac) concentrations in each voxel. If no chemical shift induced spatial displacement is present, then each voxel will measure 1.0 (a normalized figure) for each of Cho, NAA, and Lac. The spatial displacement effect distorts these values, however, and the effect is most pronounced for the voxels at the highlighted corners (upper left and lower right) in the direction of the displacement 24 (see FIG. 2). The chemical shift induced spatial displacement thus adversely effects the accuracy of MRSI chemical concentration measurements.
It is known in the art of MRSI that spectral-spatial pulses can be designed which are selective in space as well as in frequency. Spectral-spatial pulses require the use of time-varying magnetic field gradients which are synchronized with the refocusing pulse and provide the desired spatial selectivity without chemical shift selectivity.
It is also known in the art that spectral-spatial refocusing pulses can provide enhanced, accurately controllable chemical shift selectivity. More specifically, the excitation passband of spectral-spatial pulses can be made with sharp edges and can have an accurately determined width and location.